I AM standing in a cold north London workshop looking expectantly at a bizarre metal and plastic contraption. An acrid smell drifts from the machine as a length of plastic is drawn into a barrel at its centre and heated up. The molten plastic squirts from a nozzle onto a platform moving beneath it, drawing a pattern. The nozzle also moves up and down to build the design upwards like an expert cake icer.

Over the next few minutes, this "MakerBot" will do something I can only dream of doing: it will create a spare part of itself as an insurance against future mishaps. Staring at the Heath Robinson-style kit before me, it is hard to believe that it - and a few hundred other devices - are paving the way to an era of desktop machines that can make just about anything, including copies of themselves.

It could be a revolutionary age. MakerBot is one of a range of desktop manufacturing plants being developed by researchers and hobbyists around the world. Their goal is to create a machine that is able to fix itself and, ultimately, to replicate.

To find out how close we are to that goal, I have come to the London Hackspace, a communal workshop where Russ Garrett, a software developer by day, keeps his MakerBot. Like 900 other enthusiasts, Garrett bought a mail-order kit from MakerBot Industries of New York for $750, and built the machine himself.

MakerBot and most of its kin are essentially a cut-price reinvention of the 3D printer. While professional machines still cost upwards of tens of thousands of dollars, a coalition of academics and tinkerers has created versions that do much the same thing for much less. Anyone with a few hundred dollars and some spare time can build their own 3D printer from a set of plans distributed free on the internet.

These machines can build any number of things, including everything from coat hooks to ipod docking stations. One MakerBot owner even made his engagement ring with it. But key to MakerBot's popularity is its ability to make its own spares. Every kit contains a handful of parts made by other MakerBots, linking them in a mechanical family tree.

The MakerBot lineage is descended from RepRap (see "The replicant") - the first machine designed to replicate parts of itself and brainchild of Adrian Bowyer, a mechanical engineer at the University of Bath, UK. In 2006 he started the project with two goals: to create a 3D printer that anyone could make and use, and to make it capable of self-replicating. Most importantly, it would have an open-source design to encourage anyone to modify and improve it.

At the moment, RepRap can build about half of its own parts, including joints and casings. Some components, such as steel rods and microprocessors, are beyond its capabilities as yet. Still, Bowyer's mechanical progeny reached a major milestone in November 2008, when Canadian Wade Bortz announced he had used his RepRap to create all the parts of a replica that it was possible to print - the first time this had been done "in the wild" outside Bowyer's lab. It was sold online a few months later for a case of beer.

Bowyer's first design, called Darwin, has since been replaced by Mendel, which is smaller and more reliable. "Mendel can, if you discount nuts and bolts, print 50 per cent of the machine's parts in under three days," says Bowyer. Mendel can make about the same proportion of its own parts as Darwin, but Mendel is a simpler, smaller and more reliable machine. It can also make much larger things than Darwin can.

Since then, tens of others have made mothers out of their machines, sometimes selling their offspring for hundreds of dollars to other enthusiasts keen to get a machine of their own. This has led to a veritable ecosystem of RepRap-type machines - an estimated 3000 exist - and while Bowyer is now focused mainly on making Mendel more robust and user-friendly, the RepRaps in the wild have begun evolving into different forms.

While exploring the RepRap forums, I come across one with the potential to be more self-replicating than any before, and it is provoking some excited comments. The poster, Frank Davies, based in Houston, Texas, is the proud owner of a RepRap ingeniously built using parts salvaged from a dot matrix printer and a Xerox photocopying machine, and he is now working on making his RepRap totally printable.

NASA engineer's double life

Davies, by day a NASA engineer on the space shuttle programme, is effectively replacing RepRap's skeleton with one of his own making. In place of the tracks along which the print nozzle glides are plastic concertina-like mechanisms called Sarrus linkages, originally used to ensure steam pistons moved in straight lines in an era when reliably straight rods weren't available. Two perpendicular hinges connect the moving parts such that they can move only along the remaining, unrestricted axis.

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